It is common for serial data streams to be comprised of sets of contiguous frames with each frame including one or more FAS (frame alignment sequence) bits. The FAS bits constitute the FAS, and are used to delineate frames of an incoming serial bit stream and the position of the frame within a multiframe structure. Delineation of frames offers a receiving station the ability to know exactly what bit of an incoming set of frames is being received at any time.
When a receiving station is receiving frames and the location of the FAS has been determined, it is said to be In-Frame (IF). When the receiving station is not locked to the FAS, it is said to be Out-of-Frame (OOF). The receiver then must invoke a FAS search algorithm in order to find the FAS within the incoming data stream.
U.S. Pat. No. 4,347,606 to Hoogeveen which issued on Aug. 31, 1982, is entitled "Method of Frame Synchronization of a Digital TDM Communication System and Arrangement for Performing the Method" and discloses a method which keeps track of how many FAS bits, starting from the first FAS bit, have been received in each of the possible FAS bit stream locations. Each time a correct bit is received, this count is incremented. Each time an incorrect bit is received, the count is reset to zero, and the search started over again for that particular location. A disadvantage of this system is that the algorithm searches for the FAS in a predetermined order, starting with the first FAS bit. Thus, even for the correct FAS location, most of the time the embedded FAS will be out of phase with the FAS it is being compared with.
U.S. Pat. No. 4,847,877 to Besseyre which issued on Jul. 11, 1989 is entitled "Method and Apparatus for Detecting a Predetermined Bit Pattern Within a Serial Bit Stream" discloses a system in which a four bit subsequence of a twelve bit FAS is used to identify a starting point in the FAS. A table look-up is performed to identify the next eight bits of the FAS. The next twelve bits are compared to these eight bits. If these are found, then the FAS is deemed to have been located. Disadvantageously, the methods provided in Besseyre do not examine the different possible FAS bit streams concurrently and do not deal with mimics. Mimic bit streams exist when at least one potential FAS location which ultimately does not contain the actual FAS contains bits which match the entire FAS for a significant period of time.